Extraction alkali cations

For the removal of caesium from HLW to MLW, for instance, crown ethers (compounds 9 and 10, Figure 3) are well known for their ability to complex and extract alkali cations from acidic media. The stability of the complexes is linked to the relative size of the cavity of the crown ether as compared to the complexed... 

A similar study was undertaken on the related crown ether systems 201 <2001PS29>. They all showed moderate extraction of both Ag(l) and Hg(ll) ions and so were less selective than compounds 184a and 184b from the previous study. However, the presence of the benzo-15-crown-5 substituent offered the simultaneous complexation of the hard alkali cation Na(l) as well as the thiophilic metals Ag(l) and Hg(n) by the thieno sulfur. Interestingly, this second extraction was not influenced by the presence of the other metal. 

The cation selectivity in extraction experiments is dependent on differences in both the distribution constants KD and the binding constants Klf) in the water-saturated organic solvent (3). The extractability of alkali cations into... 

Similarly, a lipophilic crown ether is partitioned into the organic phase. Its complexing ability serves to transfer salts with alkali cations into the organic phase. The anions in the organic phase are poorly solvated and highly reactive. The overall reactivity in phase-transfer-catalyzed nucleophilic displacement reactions thus is a function of both the partition coefficient for extraction of the reactive anion into the organic... 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

Cryptands of type 7-9 and derivatives thereof carry alkali cations [6.4], even under conditions where natural or synthetic macrocycles are inefficient. The selec-tivities observed depend on the structure of the ligand, the nature of the cation and the type of cotransported counteranion. Designed structural changes allow the transformation of a cation receptor into a cation carrier [6.1, 6.4]. The results obtained with cryptands indicated that there was an optimal complex stability and phase-transfer equilibrium for highest transport rates. Combined with data for various other carriers and cations, they give a bell-shaped dependence of transport rates on extraction equilibrium (Fig. 11), with low rates for too small or too large... 

Extraction of Alkali Cations from Pure Water to Pure Chloroform... 

In the early 1990s, there existed several classes of extractants for actinides (CMPO), for cesium and more generally alkali cations, and for strontium and alkaline earth cations (crown ethers and cosan). The combination of these extractants and the grafting of these functions on calixarene platforms have led to new classes of extremely efficient and selective extractants, in particular calixarene-crown, which are presently applied in the United States to treat the huge amounts of waste at the SRS. Calixarenes/ CMPO, crown ethers/cosan, CMPO/cosan, and more recently calixarenes/CMPO/ cosan are promising compounds. It is desirable that these studies, conducted at the international level, continue in particular to obtain a better understanding of the complex mechanisms of extraction of these compounds.127187... 

Shinkai et al.111-151 synthesized a series of azobis(benzocrown ethers) called butterfly crown ethers , of which compounds 9 and 10 are examples. Their photoresponsive molecular motion resembles that of a flying butterfly. It was found that the proportion of their Z forms at the photostationary state increases remarkably with increasing concentration of Rb+ and Cs+, which interact with two crown rings in a 1 2 sandwich fashion. This is clearly due to the bridge effect of the metal cations with the two crowns, results that support the view that the Z forms make an intramolecular 1 2 complex with these metal cations. As expected, the Z forms extracted alkali metal cations with large ion radii more efficiently than did the corresponding E forms. In particular, the photoirradiation effect on 9 is quite remarkable for example, ( )-9 (n= 2) extracts Na+ 5.6 times more efficiently than (Z)-9 (n= 2), whereas (Z)-9(n= 2) extracts K+ 42.5 times more efficiently than ( )-9(n= 2). l ... 

Importantly, the goodness-of-fit parameters are satisfactory and the coefficient of the term involving (M +tolal) is near unity in all cases. Also, it must be noted that the stability constants for the alkali cation-benzocrown ether interaction available in the literature [30,48] agree closely with the values extracted from Fig. 5 for the alkali cation (14) association. All of this very gratifyingly support the expectations of the fluorescent PET sensor design logic in terms of a supermolecule with modular behaviour [43],... 

Alkali metal NMR in conjunction withcryptand complexation finally led to the sensational discovery of alkali metal anions by Dye and coworkers. (14) The complexing power of the cryptands is such that the ionophore is capable of extracting the cation from alkali metal in solutions of THF, methylamine, and ethylamine. The electron left behind is conclusively proved to form the anion M (M = Na, Rb, Cs) leading to a separate resonance line at low temperature. The corresponding shielding is close to the theoretical value computed for the metal anion. A most striking feature of this resonance is its solvent independence. The absence of solvent-induced chemical shifts for Na ... 

ABC extractants were found to possess attractive properties with regard to salt extraction. Extraction is efficient and selective. Compared with liquid cation exchangers, ABC extractants are less sensitive to acidity in the aqueous phase and extract alkali and alkaline earth metals better. Both the cation and the anion are extracted therefore, no acid or base addition is required for pH adjustment or for stripping. Extraction is reversible and provides for back-extraction of the extracted salt by water. Several potential applications of ABC extractants in salt extraction were studied, including MgCb recovery from concentrated seawater [2,4,5,7], separation of LiCI [6], removal of Fe from AlCl solutions [14,23], and recovery of ZnSO from zinc electrowinning bleeds [21]. 

For cation exchange, both organic polymers and very insoluble inorganic materials can be utilized as the counter phase. The basic reactions involve exchange of (most often) hydrogen ions or alkali cations for polyvalent metal ions from the aqueous (or mixed aqueous/organic) solution. Electroneutrality must be maintained in the resin phase, as it is in the less polar organic phase in solvent extraction separations, so three equivalents of monovalent cations must be released when the trivalent lanthan-ide/actinide is bound by the resin. 

Anti- and syn-isomers of 20 can extract alkali-metal cations from aqueous solutions, with binding of Cs" " by the syn-cryptophane giving the best results. The syn-cryptophane 29 has internal carboxylic acid groups that... 

The transport of alkali cations has been studied in single cation and competitive transport. A model has been developed which describes diffusion limited transport in terms of the extraction equilibria (K j) and the mean diffusion constants (D ). This model has also been used to predict the transport selectivity based on the differences in extraction ability... 

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